The Internet provides users with convenient and ubiquitous access to digital content. Because the Internet is a powerful distribution channel, many user devices strive to directly access the Internet. The user devices may include a personal computer, laptop computer, set-top box, internet enabled media player, mobile telephone, smart phone, tablet, mobile hotspot, or any other device that is capable of accessing the Internet. The use of the Internet as a distribution medium for copyrighted content creates the compelling challenge to secure the interests of the content provider. Increasingly, user devices operate using a processor loaded with suitable software to render (playback) digital content, such as audio and/or video. Control of the playback software is one way to enforce the interests of the content owner including the terms and conditions under which the content may be used. Previously many user devices were closed systems. Today more and more platforms are partially open. Some users may be assumed to have complete control over and access to the hardware and software that provides access to the content and a large amount of time and resources to attack and bypass any content protection mechanisms. As a consequence, content providers must deliver content to legitimate users across a hostile network to a community where not all users or user devices can be trusted.
Secure software applications may be called upon to carry out various functions such as, for example, cryptographic functions used to protect and authenticate digital content. In order to counter attacks, these algorithms have to be obfuscated (hidden) in order to prevent reverse engineering and modification of the algorithm or prohibit obtaining the user-specific secure information. Accordingly, the functions of the secure software application may be carried out by various functions as defined by the instruction set of the processor implementing the secure software. For example, one way to obscure these functions is by the use of lookup tables.
Content providers must deliver content to legitimate users across a hostile network to a community where not all users or devices can be trusted. This has lead to the development of white-box cryptography. In the white-box cryptography scenario it is assumed that the user has complete control of the hardware and software that provides access to the content, and an unlimited amount of time and resources to attack and bypass any content protection mechanisms. The secure software code that enforces the terms and conditions under which the content may be used should be tamper resistant. Digital rights management is a common application of secure software applications. The general approach in digital rights management for protected content distributed to user devices is to encrypt the digital content using for example, DES (Data Encryption Standard), AES (Advanced Encryption Standard), or using other known encryption schemes, and to use decryption keys to recover the digital content. These decryption keys must be protected to prevent unauthorized access to protected material.
Two main areas of vulnerability of digital rights management relying on encryption include the software modules which enforce the terms and conditions under which the content may be used, and the key distribution and handling. Typically, the software module enforces the terms and conditions under which the content is to be used. An attacker aiming to bypass these terms and conditions may attempt to achieve this through tampering of the program code of the software module.
Regarding key distribution, a media player has to retrieve a decryption key from a license database in order to play back the media. The media player then has to store this decryption key somewhere in memory for the decryption of the encrypted content. This leaves an attacker two options for an attack on the key. First, an attacker may reverse engineer the license database access function allowing the attacker to retrieve asset keys from all license databases. In this situation the attacker does not need to understand the internal working of the cryptographic function. Second, the attacker may observe accesses of the memory during content decryption, thus the attacker may retrieve the decryption key. In both cases the key is considered to be compromised.
The widespread use of DRM and other secure software has given rise to the need for secure, tamper-resistant software that seeks to complicate tampering with the software. Various techniques for increasing the tamper resistance of software applications exist. Most of these techniques are based on hiding the embedded knowledge of the application by adding a veil of randomness and complexity in both the control and the data path of the software application. The idea behind this is that it becomes more difficult to extract information merely by code inspection. It is therefore more difficult to find the code that, for example, handles access and permission control of the secure application, and consequently to change it.
As used herein, white-box cryptography includes a secure software application that performs cryptographic functions in an environment where an attacker has complete or at least substantial control of the system running the white-box cryptography software. Thus, the attacker may be able to modify inputs and outputs, track the operations of the software, sample and monitor memory used by the software at any time, and even modify the software. Accordingly, secure functions need to be carried out in a manner that prevents the disclosure of secret information used in the secure functionality.
There are various reasons why some prefer a software implementation of a cryptographic algorithm to a hardware implementation. This may, for instance, be the case because a software solution is renewable if the keys leak out, because it is has lower cost, or because the application-developer has no influence on the hardware where the white-box system is implemented. However, whitebox implementations may be found in software and hardware.
Known whitebox implementations are found in: Chow, S., Eisen, P., Johnson, H., van Oorschot, P. C., White-Box Cryptography and an AES Implementation, Proceedings of the 9th Annual Workshop on Selected Areas in Cryptography, 250-270, 2002; and Chow, S., Eisen, P., Johnson, H., van Oorschot, P. C., A White-Box DES Implementation for DRM Applications, Proceedings of the 2nd ACM Workshop on Digital Rights Management 1-15, 2002.